To me it feels more like her mother had a son after her. Causing her to be pushed aside, neglected and maybe even abused on the regular. Afraid that if she has a daughter (chance of 39/40!) her daughter would await the same faith.
Their Pixiv is: https://www.pixiv.net/user/26848/series/79030
There are also links in the description to Japanese site(s) for the full versions, but I cannot view those from my country.
And thus the story ends up with two parties:
Ceres, and the MILF party;
Hero, and the step-sister party.
Yea, this story has a lot of kinks for everybody I suppose. 😏